Magnetron



Aug. 4, 1953 J. T. RANDALL ET AL 2,648,028

MAGNETRON Filed Sept. 1'7, 1947 3 Sheets-Sheet l Aug. 4, 1953 J. T.RANDALL ET AL 2,648,028

MAGNETRON Filed Sept. 17, 1947 3 Sheets-Sheet 2 Aug. 4, 1953 J. T.RANDALL EI'AL- MAGNETRON Filed Sept. 17, 1947 3 Sheets-Sheet 3 5ounca III II III/I) ill/[1 [III fl! 11/11/72 J. 7- Randa I v Patented Aug. 4,1953 MAGNETRON John Turton Randall, Birmingham, and Henry Albert HowardBoot, London, England, assignors to English Electric Valve Company,Limited, a company of Great Britain and Northern Ireland ApplicationSeptember 17, 1947, Serial No. 774,596 In Great Britain August 22, 1941Section 1, Public Law 690, August 8, 1946 Patent expires August 22, 1961This invention relates to high frequency electrical oscillators and moreparticularly to oscillators of the magnetron type. We have shown amagnetron type of oscillator in our prior copending application SerialNumber 407,680, filed August 20, 1941, now U. *8. Patent 2,542,966granted February 20, 1951, and the present invention is addressedprimarily to improvements in the type of oscillator therein disclosedalthough the invention may be applicable to certain other oscillators.The type of magnetron to which this invention is particularly applicablecomprises a metallic anode block having a central cavity with aplurality of resonator cavities around the central cavity and havingopenings into the central cavity. A cathode is positioned along the axisof the central cavity. The magnetron is particularly useful inpulse-echo systems more commonly known as radar systems, although it isapplicable to other radio frequency oscillators which emit waves thelengths of which are in the order of a few centimeters. It is oftendesirable in oscillators operating on the wave lengths referred to, forthe power output to be in the form of a series of pulses separatedbysubstantial intervals.

In the magnetrons of the type referred to it has been usual hitherto tomake use of the thermionic emission, the cathode being either directlyor indirectly heated and having in most cases a suitable coating for thepurpose of producing a more powerful emission than would otherwise beobtainable. Now it has been found that in apparatus developingconsiderable power such cathodes have tended to become over-heated. andthis has set a limit to the output powers obtainable. The primary objectof the present invention is to overcome this and other disadvantagesresulting from the use of thermionic emission, such as the necessity forproviding a heater element and its associated circuit, by the provisionof an alternative means of obtaining the necessary electronic emissionfrom the cathode. Other objects include the provision of a magnetronthat can be conveniently pulsed.

According to the invention, a high frequency electrical oscillator ofthe specific magnetron type referred to is characterized in that thephenomenon of secondary emission is utilized for the production of thewhole or major part of the main electronic stream from the cathode. Bysecondary emission is meant the known effect whereby the bombardment ofa surface by an electronic stream results in the emission from thissurface of further secondary electrons 15 Claims. (Cl. 315-40) whosenumber may considerably exceed that of the primary or bombardingelectrons. The average number of such secondary electrons released byone primary electron is termed the secondary emission coefficient, afactor which varies with the nature of the emitting surface and thevelocity and angle of impingement of the bombarding electrons. Accordingto a preferred feature of the invention a certain percentage of thesecondary electrons leaving the cathode are caused to return thereto andact as primary or bombarding electrons, the process of secondaryemission thereby being maintained.

The essential factors for the production of secondary emission from a'cathode in accordance with the invention are: (1) a suitable emittingsurface (which may, as hereinafter described, be of a very simplenature) (2) the provision of primary electrons, and (3) the provision ofmeans for bringing about the bombardment of the oathode by the saidprimary electrons. These factors will be considered in turn.

As regards factor 1) almost any conducting or semi-conducting surfacewill exhibit the effect, but it has been found that very satisfactoryresults are obtainable from the use of a thin semiconducting (e, g.oxide) film on a metallic surface. In such cases the effect is believedto be enhanced by what is known as thin film field emission or theMalter effect: the current hypothesis concerning this effect is thatsecondary emission from the outer surface of the thin semiconductingfilm leaves positively charged areas on this surface, and these positivecharges, separated from the main conducting surface only by thethickness of the film, produce a powerful field which causes theemission of further electrons therefrom.

Suitable emitting surfaces consist of thin coatings of the oxides ofthorium or aluminum on a suitable metallic base. Calcium oxide and thesilicates of barium (3Ba0 1Si02 and 2BaO 18102) are also suitable; thesesubstances will also work as primary emitters. Good effects have alsobeen obtained by the use of a metallic cathode coated with the oxides ofbarium or strontium, such as are used for ordinary thermionic cathodes.A coating ofcaesium on an emitting surface is known to produce a highsecondary emission coefiicient, and accordingly such a coating may beemployed on a cathode for the purpose of the present invention; asuitable cathode may comprise, for example, a coating of caesiumsuperimposed on a thin oxide film formed on a suitable metallic base.

The choice of the emitting surface employed will depend upon therequirements of individual cases. If, for 'exampla'a high emissioncurrent is required for a relatively small number of primary orbombarding electrons, it may be desirable to choose a substance such ascaesium having a high secondary emission coefficient; if, however, anadequate supply of primary electrons is available, a particularly highsecondary emission coefficient is no longer of paramount importance, andit may be preferred to use a substance having a relatively lowcoeificient of secondary emission provided that this substance has someother quality or qualities which make its use desirable in theparticu1ar case in question. Thorium oxide (ThOz), for example, isparticularly suitable in cases where a heavy emission current isrequired, since it is non-volatile and will, therefore, stand up to aheavy back-bombardment.

The selection of the base material on which the emitting film isdeposited is likewise governed by the design considerations of theparticular case. Good results have been obtained with aluminum as abase, either in the form of a member of aluminum or a copper memberhaving an aluminum coating formed thereon in known manner. In caseswhere a heavy current is involved and there is likely to be considerableheating of the cathode it is, however, preferred to use a metal with ahigh meltingpoint such as molybdenum or tantalum. A thorium oxidecoating on a molybdenum base forms a particularly suitable combination,from which a large secondary emission may be obtained at a much lowertemperature than if it were used solely as a primary emitter. It will beunderstood, however, that any of the secondary emission coatingsenumerated above may be used with any of the above-mentioned materialsor with any other suitable base.

In certain cases the emitting surface. may consist of a film of theoxide of the base metal itself, which may then be formed directly on thebase. Thus a form of cathode which has been found satisfactory consistsof a freshly machined aluminum surface which is cleaned in caustic sodasolution and subsequently heated in boiling water to accelerate theformation of a thin oxide film. Beryllium-copper alloys have been foundto work as secondary emitters, probably as a result of the formationthereon of a thin film of beryllium oxide.

Regarding the provision of primary electrons, it has already been statedthat according to a preferred feature of the invention these areprovided by a certain percentage of the secondary electrons which havealready left the cathode. It is, however, still necessary to provideprimary electrons in the first instance to initiate the secondaryemission effect from the cathode when first switching on.

The most efiicient means for providing such primary electrons appears tobe the use of thermionic emission, either from the main cathode itself(which in this case must beinitially heated) or from an'auxiliary orpilot cathode. When the pilot cathodeiis employed, the usual heaterinside the main cathode may be omitted.

Such a pilot cathode may comprise a simple tungsten filament, but it isfound that the necessity for keeping such a filament at a sufficiently.high temperature to ensure adequate primary emission tends to lead toan erratic life performance. It is preferred, therefore, to use a pilotcathode suitably coated with emitting material so that it will operateat a lower tempera-" ture, say 1500 0.; this is preferably arranged at asmall distance (say 1 mm.) from the main cathode. The oxides of bariumor strontium (or thorium oxide used as a primary emitter) form suitablecoatings for such a pilot cathode, from which an adequate supply ofprimary electrons may be obtained to enable relatively heavy secondaryemission currents to be produced from the main cathode, even though acomparatively poor secondary emitter is used thereon. It is found thatthe area of the emitting surface of the pilot cathode is sufiicientlysmall to prevent it from becoming overheated in operation.

As an alternative to the provision of a separate pilot cathode the maincathode may be provided with a heater, and the ends (which are coolerare not so subject to the effects of back bombardment as the centralportion) provided with coatings of suitable thermionically emittingmaterial. The coated ends may each occupy roughly a tenth of the totallength of the cathode, e. g., 3 mm. at each end of a 3 cm. cylindricalcathode. As a further alternative a directly heated main cathode may beemployed, comprising, for example, a wire helix. If desired a heatedmain cathode may be used in conjunction with a pilot cathode, thefunction of providing heating means for the main cathode being in thiscase to effect the degassing of the apparatus and/or to providesupplementary thermionic emission as hereinafter described.

An alternative source of free electrons for the initial bombardment ofthe cathode may be provided by the ionization of any low-pressure gas orvapor within the discharge apparatus when the H. T. supply is switchedon, and it appears that even in very hard tubes the minute quantity ofresidual gas is adequate to produce this effect provided a sufficientlyhigh anode voltage is employed. The disadvantage of this method is thetendency of such residual gas to clean up" after the apparatus has beenin use for some time. As a further alternative a source of ionizingradiation may be arranged either internally or externally with respectto the discharge apparatus; a small quantity of radioactive materialmay, for example, be provided within the apparatus.

A convenient means for effecting the necessary bombardment of thecathode is provided by the magnetic field which is normally utilized forthe operation of the magnetron, the direction of this field beingsubstantially at right angles to the direction at any point of theelectrostatic field between the anode and the cathode. It is known thatan electron moving in a vacuum and acted on solely by a uniform magneticfield whose direction is at right angles to its plane of motion willtheoretically traverse a circular path whose radius is directlyproportional to the velocity of the electron; if an electric field isalso present, however, the path will take a general curved form whoseexact shape depends on the relative strengths and directions of the twofields.

Considering now the free electrons initially produced in apparatusaccording to the invention by any of the means previously described, itwill be seen that under the action of the applied electrostatic fieldsuch electrons will commence to move towards the anode; the presence ofthe magnetic field will, however, cause them to follow curved paths, andsome of them will thus strike the cathode. Secondary electrons will thusbereleased, and of these a certain number (that is, those having asufficiently small initial velocity) will, under the action of themagnetic field, traverse paths whose curvature is sufiiciently great tocause them to return to the cathode. These in turn will cause therelease of further secondary electrons, and the secondary emission willthus build up automatically until an equilibrium condition, dependentupon the space charge distribution, is reached. Of the total number ofelectrons emitted by the cathode a certain percentage thus return tomaintain the process of secondary emission, while the remainderconstitute the main electron stream required to carry out the functionof the apparatus.

The magnetic field may be provided by any suitable permanent magnet,electromagnetic or solenoid arranged either externally or internallywith respect to the discharge apparatus. In a simple form the cathodemay itself constitute a permanent magnet which provides the neces saryfield.

As an alternative to the use of a magnetic field for th production ofthe bombardment effect an electrostatic field may be employed, rovided,for example, by a suitable grid or auxiliary electrode to which a steadyor alternating potential is applied for the purpose of causing apercentage of the electrons emitted by the oathode to return thereto asdescribed above.

In the operation of a cathode according to the invention the secondaryemission effect may be supplemented by thermionic emission, as forexample, by the use of a main cathode provided with a heater asdescribed above. Apart from the question of starting up, however, it isnot necessary to provide a heater for the purpose of obtainingsupplementary thermionic emission from the main cathode, since thenecessary temperature may b attained by the normal heating up of thecathode during operation. Since a heating element is not essential in asecondary emission cathode, such a cathode is readily adapted to beairor water-cooled for the purpose of preventing it from heating upduring the operation of the apparatus, or limiting the rise oftemperature in the case where a certain amount of heating up is allowedto provide supplementary thermionic emission. The cathode may, forexample, consist of a simple metal tube whose exterior may be suitablycoated and through which water may be assed for cooling purposes; acopper tube having a coating of aluminum which in turn has a coating ofaluminium oxide may conveniently be employed, this constructionfacilitating the soldering on of copper inlet and outlet tubes.

An important point to observe in connection with a secondary emissioncathode having no heater is that it cannot be degassed in the normalmanner; in high vacuum apparatus the cathod must therefore be keptreally cold during operation, e. g., by water cooling, otherwise theapparatus may tend to become gassy after a short running period. Thealternative is to use a cathode provided with a heater as described;this may either be switched off during operation or kept on to providesupplementary thermionic emission according to circumstances, but in anycase the use of secondary emission enables the cathode to be run at amuch lower temperature than if thermionic emission alone were employed.

It is contemplated that all of the foregoing cathode arrangements areadapted to be employed in magnetrons which are to be pulsed. Inconnection with all of the several modifications mentioned above, pulsepower is applied to the magnetron; and at the beginning of each pulse,the auxiliary source of electrons starts the flow of primary electrons.

Specific constructions according to the invention will now be describedby way of example with reference to the accompanying drawings in which:

Figure l is a longitudinal cross-sectional view of a secondary emissioncathode suitable for use in amagnetron and provided with a heater and apilot cathode,

Figures 2 and 3 are end views of the cathode illustrated in Figure 1,looking in the directions of the arrows A and B respectively.

Figures 4 and 5 are respectively a longitudinal cross-sectional view anda medial transverse cross-sectional view of a magnetron according to oursaid prior copending application and incorporating a water-cooledsecondary-emission cathode,

Figures 6 and 7 are fragmentary views (longitudinal and transversecross-sections respectively) ofa water cooled cathode, drawn to aslightly larger scale than the cathode shown in Figures 4 and 5,

Figure 8 is a schematic diagram of the electricalconnections employed inconnection with the magnetron of Figures 1 to '7 inclusive,

Figure 9 is a cross-sectional view of a cathode such as may be used withour invention, there being also shown a radioactive substance forestablishing primary electrons,

Figure 10 is a schematic diagram of a modified form of our inventionwherein a screen is employed to cause anyfree electrons leaving thecathode to return thereto and thereby establish secondary emission,

Figure 11 illustrates a modified form of our invention wherein ,a veryhigh anode voltage causes ionization of the gas in the tube and therebyestablishes primary electrons, and

Figure 12 is a cross-sectional view of a modified form of this inventionin which thermionic emission takes place at the ends of the cathode tothereby establish primary electrons.

Referring first to Figures 1 to 3, the cathode shown comprises amolybdenum cylinder I having a coating of a suitable secondary emittersuch as thorium oxide deposited thereon. The pilot cathode comprises afilament 2, coated with thorium oxide, and arranged parallel with themain cathode and at a distance of the order of 1 mm. from its surface.The filament 2 is mounted in an insulating lug 3 at one end and issecured to a spring 4 at the other end for tension ing purposes. Themain cathode is provided with a heater coil 5 for the purpose ofdegassing and/or the provision of supplementary thermionic emissionduring operation as previously described. Nickel end shields 5' serve tosupport the mountings of the heater 5' and the pilot filament 2, and toprevent the passage of stray electrons beyond the ends of the maincathode. The heater 5 may, of course, be omitted entirely when othermeans of starting flow of primary electrons is employed.

Figures 4 and 5 show a magnetron of the type covered by our said priorcopending application. The apparatus comprises a main anode block 6having a plurality of resonators l in the form of cylindrical cavitiesdrilled therein. These resonators have narrow longitudinal gaps 8opening into a central space 9 in which the cathode is located, and theresonators and the central spaceopen at both ends into end space 10. Theoutput power is transmitted through a coupling loop inserted in oneoftheresonators.

The secondary emission cathode according to the present invention is ofthe water cooled type and comprises a .hollow cylinder |2 provided witha suitable coating and having copper tubes l3 soldered or otherwisesecured to its ends whereby water may bepassed through the. cathode forcooling purposes. The tubes |3, which also form the cathode connection,pass out laterally through the end spaces ID of the magnetron, beingsupported and insulated from the main block 6 by glass tubular membersIt sealed in known manner to copper tubular members l5, I6 which aresoldered to the main block 6 (over suitable apertures) and to the tubes|3 respectively.

The pilot cathode of Figures 3 and 4 comprises a filament suitablyconnected at one end to the main cathode I2 and connected at the otherend to a lead l8 which passes out through a glass cap l9 sealed to acopper tubular member 20. A similar copper-glass seal 2| is used for theoutput lead from the coupling loop As shown in more detail in Figures 6and '7 the pilot cathode I1 is inset into a longitudinal depression 22formed in the surface of the main tubular cathode l2, the wall of whichis made sufliciently thick to have the said depression cut therin (asshown) or which may alternatively be indented to form the depression.

Secondary emission cathodes according to the invention are capable ofproducing high emission currents, and are therefore suitable for use inmagnetrons developing considerable power. Moreover, a high space chargedensity may be formed around the cathode, and this is found inmagnetrons of the type referred to above to lead to high efiiciencies. Afurther advantage of the invention resides in the fact that thepossibility of eliminating the heater element provides a simpler andmore robust construction than is possible with thermionic cathodes, aswell as removing a potential source of breakdowns.

Cathodes according to the invention may be of cylindrical, disc or anyother form according to requirements.

A main magnet (or other flux producing means) is employed to effect afield parallel to the main cathode in all forms of our invention. InFigure 4 the magnet poles set up the desired field.

Since one of the advantages of the magnetron which we have hereinbeforedescribed is I that it is especially suitable for pulse transmitters, weshall proceed to describe one circuit arrangement for so energizing themagnetron as to produce a pulse output. In Figure 8, there is shown themagnetron tube of Figures 1 to '7 inclusive in which the pilot cathodeI1 is heated to electron emitting temperature by current from thesecondary of filament transformer 30. A source of pulse power 33 has itsnegative pole connected to the pilot cathode I1 and its positive pole tothe anode block 6. The cathode I2 is connected to pilot cathode I! atone end. The source of pulse power 33 is preferably any device forproducing sharp pulses. Alternatively it may be the secondary of a highvoltage alternating current transformer in which event it will energizethe magnetron on alternate half cycles and thereby produce'a modulatedwave output. Any other device for producing interrupted continuous waves(I. C. W.) such as for a chopper may be employed. In any event when thepulser 33 produces a pulse of the polarities shown on the drawing, thepilot cathode emits electrons which due to the main magnetic field takea curved path and some of them bombard the cold cathode l2 therebyefiecting secondary emission. The high potential of theanode 6 causeselectrons to leave pilot cathode H. The field of the main magnet .poles"l5 deflect the electrons leaving pilot cathode I"! and therefore theydo not strike the main cathode 2 solely at one place near the pilotcathode but are distributed about the main cathode l2. The cold cathodeI2 is negative with respect to anode 6 and therefore the secondaryelectrons are emitted initially toward anode 6, however they assume'acurved path due to the main'magnetic field which is parallel to the axisof cathode l2. The electrons after leaving the cold cathode|2 graze thepole pieces of the anode 6 thus setting up circulating currents in theresonator cavities The loop H (see Figure 5) is arranged to transfer thegenerated powerjtothe antenna circuit. At the conclusion of each pulseinitiated by pulser 33, the anode to cathode potential, as well as thepotential between the anode 6 and the pilot cathode I1, is stoppedthereby stopping generation of further powerby the magnetron.

Figure Qillustrates an alternate system for establishing primaryelectrons. The cold cathode '|2 supports an ionizing element 40 whichmay be a radium coated element. The inside of tube |2 may form part of awater cooling system.

Figure 10 illustrates still another system for establishing primaryelectrons. A cold cathode 50 is separated from the anode pole pieces 53.Positioned intermediate the cathode 50 and the pole pieces 53, we locatea screen comprising a series of wires 5| that are parallel to the coldcathode 50. These parallel wires 5| are interconnected by a wire .52.Wires 53 and 54 respectively connect the cold cathode 50 and thescreen'5l to a source of current 59 which source may be either steady oralternating. A source of direct current 55 has its negative poleconnected to the cold cathode" and its positive pole connected to anodeblock 6. The operation of Figure 10 depends upon the free electrons onthecold cathode 50. These free electrons are attracted by the highpositive potential of the anode 6 but fail to reach the anode block 6since they are caused toreturn to the cathode 50 by the negative chargeon the screen 5|. Hence the cold cathode-50 is bombarded by primaryelectrons. The magnetron operation may be controlled by regulating thepotential of source 59.

In Figure'll the small quantity of gas that remains in the tube afterevacuation is ionized by the very high voltage from source 64 which isconnected by wires 62 and 63 across the cathode BI and the anode;60. Theresult of this ionization is a bombardment of the cathode I2 and theemission of secondary electrons. If the high voltage supply 84 ispulsed, the output current will also be pulsed.

In-Figure lZ there is shown a thin tube having the usual oxide coatingadapted for secondary emission thereon. The total length of the cathode80 may be 3 centimeters. One or both ends of the tube 80 may havecoatings 82 which each extend for approximately one-tenth the totallength of tube 80. Coatings 82 are suitexample able for thermionicemission and are provided with suitable heaters which may be eitherinside or outside tube 80 but are specifically illustrated in Figure 12as heater coils 8| inside of tube 80; To start the magnetron, theheaters 8| are energized to cause emission of primary electrons fromcoatings 82. Some of these electrons strike the outside layer on tube 80and cause secondary emission, which will from that point build up byitself.

It should be clearly understood that as for any features notspecifically referred to herein that the devices shown in thisspecification are similar to the teachings of my said prior copendingapplication. For example, said prior application teaches that thediameter of the cathode may be about 0.40 the inside diameter of thecentral cavity. This is true in the present invention;

We claim:

1. In a magnetron, an anode block defining a central cavity and aplurality of resonator cavities opening into the central cavity, a coldcathode comprising a thin cylindrical tube positioned axially in thecentral cavity, a coating on the outside of the tube for establishingsecondary emission, means for passing cooling fluid through the insideof said tube to thereby cool the tube, and means for effectingbombardment of the outside of the tube with primary electrons to therebystart the magnetron into oscillation.

2. In a magnetron, an anode block defining a main cylindrical cavity anda plurality of resonator cavities opening into the main cavity, asubstantially cylindrical secondary emission surface coaxial with aswell as coextensive with the main cavity, a primary thermionic emitterfor bombarding said surface to excite secondary emission therefromcomprising a filament wire positioned between said cathode and saidanode, means for connecting a source of current to opposite ends of saidwire and thereby to heat the wire to electron emission temperature,means for establishing a magnetic field in the main cavity and coaxialtherewith, and means for establishing a high potential between saidcathode and said anode block.

3. The magnetron defined in claim 2 in which said wire is positionedcloser to the cathode than to the anode and is at all points equallyspaced from the cathode.

4. The magnetron defined in claim 3 in which the wire is electricallyconnected to the cathode.

5. The magnetron defined in claim 2 in which the Wire is electricallyconnected to the cathode.

6. The magnetron defined in claim 2 in which the cathode defines anelongated indent in the surface thereof, said indent being substantiallyparallel to the axis of the cathode, and means supporting said wireparallel to and closely adjacent the indented surface of the cathode andcloser to the cathode surface than to the anode surface.

7. In a magnetron, an anode block defining a cylindrical cavity thereinand a plurality of resonator cavities opening into the main cavity, anevacuated envelope surrounding the anode block, a hollow cylindricalcathode coaxial with the main cavity, two water pipes connected to theopposite ends of said cathode and extending respectively perpendicularlytherefrom through the envelope, means on the surface of said cathode forestablishing secondary emission upon bombardment by primary electrons, athermionic emitter for bombarding said surface with primary electrons,means for establishing a magnetic field 10 parallel to said axis, andmeans for applying a potential between the anodeand the cathode of suchpolarity as to render the anode positive.

8. In a magnetron, an anode block defining a main cylindrical cavity anda plurality of at least three resonator cavities separately opening intothe main cavity, a secondary emission type of cathode located in themain cavity and along the axis thereof, the openings from the resonatorcavities into the main cavity all intersecting a single plane thatisperpendicular to the axis of and passes through said cathode, meansfor establishing the main magnetic field for the magnetron comprisingmeans for setting up a strong magnetic field parallel to the axis ofsaid main cavity, a source of potential for charging the anodepositively with respect to the cathode, means adjacent the cathode forstarting secondary emission by bombardment of the cathode by electrons,said magnetic field and said potential being of such magnitudes as tocause the electrons in the space between the cathode and anode to movein curved paths and such that some of the electrons in that space willmove toward the cathode and bombard the latter.

9. In a magnetron, an anode block defining a main cylindrical cavity anda plurality of at least three resonator cavities symmetrically locatedaround the main cavity and opening into it,'a cylindrical cathodecoatedwith a surface adapted for secondary emission, said cathodeextending coaxial of the main cavity and substantially the entiredistance between opposite faces of said block, a thermionic emittercomprising a member heated to electron emission temperature, saidemitter being located adjacent said cathode whereby to bombard thelatter, means for establishing a magnetic field parallel to the axis ofsaid main cavity to operate the magnetron thereby causing electrons inthe cathode-anode space to establish oscillations in the resonatorcavities, and a source of potential connected between the cathode andthe anode and arranged to charge the anode positively with respect tothe cathode, said potential and said field being of such magnitudes asto cause some of the electrons emitted by said emitter and said cathodeto move in a curved path about the outside surface of the cathode and tostrike the cathode and thereby excite secondary emission from thecathode.

10. The device of claim 9 in which said cathode is hollow, a pair ofpipes respectively connected to opposite ends of said cathode, and meansfor forcing a cooling fluid into one of the pipes, whereby to cool thecathode.

11. In a magnetron, an evacuated envelope having an anode block therein,said anode block defining a main cavity and a plurality of at leastthree resonator cavities therein, a secondary emission type of cathodelocated substantially in the center of the main cavity, a pilot cathodelocated nearby the first-named cathode, means for creating a mainmagnetic field transverse to the cathode-anode path, a source ofpotential between the cathode and anode, said potential and said fieldhaving such magnitudes as to cause some of the electrons in thecathode-anode space to take curved paths and then strike the first-namedcathode thus bombarding it and heating it, said first-named cathodebeing of very large area as compared to that of the pilot cathode suchthat the first-named cathode can supply substantially all of theelectrons involved in operation of the magnetron and whereby the pilotcathode will not be overheated in operation,

11 and a current path: for heating the pilot cathode, said current path:providing substantially the entire heating efiect on the pilot cathode,and electron bombardment constituting substantially the entire heatingof theflrst-named cathode.

12. The magnetron defined by claim 11 in-' cluding in addition coolingmeans for coolin the first-named cathode without substantially coolingthe pilot cathode.

13. The magnetron defined by claim- 12- in which the pilotcathode-comprisesan oxide-coated filament designed to operate normallyat approximately 1500 degrees centigrade.

14. In a magnetron, ananode structure defining a cylindricalcentralcavity anda plurality of resonator cavities opening into thecentral cavity, a tubular cathode positioned axially in said centralcavity andflhaving an outer surface which emits electrons when'bombardedby electrons, means adjacent the cathode for carryingaway heat fromthe-inside of the tubular oath ode comprising means forpassing a coolingfluid through the inside of the cathode, and means for establishingprimary electrons-in the space between the anode and the cathode tothereby start the magnetron.

15. The magnetron as defined in claim 14 in which the last-named meanscomprises a thermionic emitter located in the spacebetween the anodevand the. cathode, andth'e inner wall of the tubular cathode constitutingthe only wall which confines the cooling fluid inside of the cathode.

JOHN TURTON RANDALL. HENRY ALBERT HOWARD BOOT.

References Cited in the file of this patent UNITED STATES PATENTS NumberNumber Name Date Samuel Dec. 8, 1936 Samuel 2- June 20, 1939 Hollman May21, 1940 Hansell Oct. 15, 1940 Skellett June 3, 1941 George Sept. 8,1942 Hansell Oct. 8, 1946 Spencer Nov. 26, 1946 FOREIGN PATENTS CountryDate Great Britain July 11, 1939 Great Britain Nov. 19, 1946 GreatBritain May 16, 1947 Great Britain May 16, 1947 France June 26, 1939France Nov. 16, 1937

